Using a plasmid with a cloned selectable gene, but no autonomously replicating sequence, we have begun analysis of the mechanism of transformation of Ustilago maydis. Plasmid DNa in the circular form was found to transform an auxotrophic strain to prototrophy by integration into the cellular genome primarily at nonhomologous loci. When cut within the cloned selectable marker gene, the plasmid was found to integrate into the genome for the most part at the site of the homologous resident allele. Thus, DNA conformation directs the mole of plasmid integration. Gene conversion was observed when circular DNA was used, but was not associated with crossing over. A novel extrachromosomal mode of transformation was discovered in which transforming DNa was propagated as a large concatameric array of direct and inverted repeats. In this proposal we plan to investigate the genetic control of these pathways of transformation by examining how different genetic backgrounds recombinogens, and non B-form DNa sequences influence the frequency and mode of recombination. Additional plasmid substrates will be developed as means to measure the frequency of recombination and to serve as indicators in screening for mutants defective in recombination.